I've personally owned maybe six different 67-72 trucks with 4-6 drop and I have never notched any of them or felt the need to. And, I drive them plenty. Worst case, you hit a railroad crossing at 40 miles per hour and you might hear it thump against the bump stops.

Look up Make It Handle by Rob at NO LIMIT . This is full of info every truck owner should read .He says that if you bottom out the rear against the frame you only have about 3/4 to 1 inch clearance between the top of the rear end and the bed floor depending on wood or steel floor. If you c-notch the frame more than 1 inch you will need to cut your floor fore any more drop.

Look up NO LIMIT .NETon the Interweb and look at their relocation kits.My truck has bags in place of coils for a 6in drop I cut my snubbers in half and empty it takes a LOT to bottom out The best thing I did was use their panhard bar and shock kit.Drives like a completey different truck.Best bang for the buck IMHO!

I agree with everyone on the rear, but no one has addressed the front yet. I have 4" springs in front and you are definitely going to need to make some cuts if you go that route. At 4" spring drop, you are just sitting on the bump stops. I had to completely cut out the bump stop mount and put in a smaller bump stop to get any decent travel up front. Also, your shock is going to be bottomed out bad at a 4" spring drop. I custom made and welded in a new top shock mount to give me adequate travel. There are aftermarket top shock mounts that will work, but I wanted to make my own. If you don't want to go through all this trouble, I recommend installing some 2.5" drop spindles and either a 1" or 2" drop spring. That will get you where you want to be and you wont have to cut anything.

Basically I want to go as low as possible without cutting the frame or do any modifications to the frame.
Because of the VIN number and the desirability of the truck in the future
I don't want to do aything to the frame since it is a virgin frame, and all matching numbers.

If anyone hows how to accomplish this or knows of ways to get as low as possible without bastardizing the frame please let me know.